Bioethanol production from renewable feedstock by baker's yeast Saccharomyces cerevisiae has become an attractive alternative to fossil fuels. However, the availability of starch or sucrose based feedstock such as corn grain or sugar cane is expected to be insufficient to cover future worldwide needs for bioethanol (Gray et al., 2006. Bioethanol. Current Opinion Chemical Biology. 10(2):141-146). A foreseen solution is the utilization of lignocellulosic feedstocks, such as corn stover, wheat straw, sugar cane bagasse, wood, etc (Hahn-Hägerdal et al., 2006. Bioethanol—the fuel of tomorrow from the residues of today. Trends Biotechnol. 24(12):549-556). This requires overcoming new challenges associated with the utilization of these complex raw materials.
A substantial fraction of lignocellulosic material consists of pentoses, xylose and arabinose that need to be efficiently converted to make the bioethanol process cost-effective. Saccharomyces species cannot ferment these pentoses as such and need to be modified to be able to do that. However, attempts have been made to modify Saccharomyces strains to produce ethanol and other fermentation products such as butanol, lactate, 1,4-diacids (succinate, fumaric, malic), glycerol, sorbitol, mannitol, xylitol/arabinitol, L-ascorbic acid, xylitol, hydrogen gas, 2,5-furan dicarboxylic acid, 3-hydroxy propionic acid, aspartic acid, glutaric acid, glutamic acid, itaconic acid, levulinic acid, and 3-hydroxybutyrolactone in an efficient way. Saccharomyces cerevisiae, which can be grown on xylose aerobically and which ferments xylose to ethanol has been obtained, wherein said strain either has genes from the Pichia stipitis xylose pathway or heterologous xylose isomerase (XI) genes and overexpresses the endogenous xylulose kinase gene (Hahn-Hägerdal B, Karhumaa K, Fonseca C, Spencer-Martins I, Gorwa-Grauslund M F (2007). Such strains do not grow anaerobically on xylose as sole carbon source. However, anaerobic growth is a crucial trait for industrial fermentation processes since it renders the yeast viability and viability is directly related to the ability of the yeast to ferment efficiently. Anaerobic xylose growth by recombinant strains of S. cerevisiae has been achieved in haploid laboratory strains by random evolutionary engineering strategies (Sonderegger M, Sauer U (2003) Evolutionary engineering of Saccharomyces cerevisiae for anaerobic growth on xylose. Appl Environ Microbiol 69:1990-8; Kuyper et al (2004) Minimal metabolic engineering of Saccharomyces cerevisiae for efficient anaerobic xylose fermentation: a proof of principle. FEMS Yeast Res 4:655-64). Traits obtained via random strategies are not easily identified, and therefore such traits are difficult to transfer to other strains. Moreover laboratory strains do not ferment toxic lignocellulose hydrolysates (Karhumaa K, Garcia Sanchez R, Hahn-Hägerdal B, Gorwa-Grauslund M F (2007) Comparison of the xylose reductase-xylitol dehydrogenase and the xylose isomerase pathways for xylose fermentation by recombinant Saccharomyces cerevisiae. Microb Cell Fact 6:5).
Furthermore, when applied to polyploid and aneuploid industrial strains random strategies often result in limited improvements. Therefore there is still a need to design well-defined rational metabolic engineering strategies/technologies, which convey traits that provide Saccharomyces sp. strains with the ability to grow and ferment pentose sugars anaerobically and which can be transferred to any other polyploid and aneuploid Saccharomyces sp. strain.